Known is a multi-lead screw gerotor mechanism for a screw downhole motor, comprising: a stator having inner helical teeth made of a resilient-elastic material, e.g. of rubber; and a rotor having outer helical teeth, number of which outer teeth by one tooth is less than that of the stator teeth; the rotor axis being shifted with respect to the stator axis by the eccentricity value being half of the teeth's radial height; profiles of the rotor s outer teeth and stator's inner teeth are mutually-enveloping when viewed in the end-face section; and leads of the rotor and stator teeth being proportional to a number of their teeth (see patent RU 2165531, IPC P01C 1/16, 5/04, E21B4/02, 2000).
In the prior-art designs, profiles of the stator and rotor teeth, when viewed in the end-face section, are implemented as the envelopes of the common initial contour of the cycloidal rack defined by the curtailed cycloid equidistance. In this end-face section, thickness Ct of the stator tooth across the mean diameter Dm of the teeth and circular pitch St of these teeth are interrelated according to the following ratio: Ct/St=0.45 0.65; and thickness CN of the stator tooth across the mean diameter Dm, when viewed in the section perpendicular to the stator tooth's screw line direction, and the stator's tooth radial height h are interrelated according to the following ratio: CN/h≧1.75.
A drawback of this known gerotor mechanism consists in that the total diametric interference in the mechanism is distributed among the stator teeth in the manner that the stator tooth projection is deformed significantly more than its space, so that the rotor axis may shift toward the eccentricity decrease and, consequently, the designed kinematics of the gerotor mechanism may be departed from, wear of apices of the rotor and stator teeth may become more intense, the interference in the pitch point zone may weaken, and service life of a gerotor mechanism may become briefer.
Said drawback is partially mitigated in the gerotor mechanism, comprising: a stator having inner helical teeth made of a resilient-elastic material, e.g. of rubber; and a rotor having outer helical teeth, number of which outer teeth by one tooth is less than that of the stator teeth; the rotor axis being shifted with respect to the stator axis by the eccentricity value being half of the teeth's radial height; leads of the rotor s and stator's helical teeth are proportional to numbers of their teeth [patent RU 216603, IPC E21B 4/02, 2000].
The stator teeth's profile, when viewed in the end-face section, is implemented as the envelope of the initial contour of the cycloidal rack defined by the equidistance having radius RC1 of the curtailed cycloid; and the rotor teeth profile, when viewed in the end-face section, is implemented as the envelope of the outer initial contour of the cycloidal rack having radius RC2 of equidistance, which radius is greater than RC1 or obeys the following ratio: RC2=RC1+(0.1 . . . 0.5) E, where E is the generating circle radius being equal to the eccentricity value [see said patent No. 2166603].
Another version of said know design of a gerotor mechanism is such that the stator teeth's profile, when viewed in the end-face section, is implemented as the envelope of the initial contour of the cycloidal rack defined by the equidistance having radius RC1 of the curtailed cycloid; and the rotor teeth's profile, when viewed in the end-face section, is defined by the conjugated circular arcs; the rotor tooth's projection being defined by arc of radius RB, which radius is greater than radius RC1 of the stator equidistance, or interrelates with said radius according to the following ratio: RC2=RC1+(0.1 . . . 0.5) E, and the rotor tooth's space profile is defined by the arc having radius RV, which radius depends on a number of rotor's teeth, inner diameter and eccentricity of said rotor (see said patent No. 2166603].
A drawback of the above-recited design is as follows: as the lateral and diametric interferences, evenly distributed, take place, high contact stresses arise and reach their maximum at minimal angles of pressure, which results in one-sided frictional wear of the teeth (at the left side of the rotor teeth, when viewed from the working fluid delivery side), and the friction forces, that develop in meshing, bring about the moments of resistance that prevent the rotor from rotating about its axis and from its planetary motion, which circumstances impair the energy characteristics of a given mechanism.
The device most pertinent to the claimed invention is a multi-lead gerotor mechanism of a screw hydraulic motor, comprising the following constituents: a stator having inner helical teeth made of a resilient-elastic material, for example of rubber; and a rotor having outer helical teeth whose number is one tooth less than that of the stator's teeth; the rotor axis having been shifted with respect to the stator axis by the eccentricity value being equal to half of the teeth's radial height, the end-face profile of teeth of one of the constituents is implemented as the envelope of the initial contour of the rack defined by the curtailed cycloid equidistance with a shift; and the end-face profile of teeth of the other constituent is implemented as the equidistance of envelope of the first constituent when their centroids are revolved around without slippage, and the equidistance value being half of the value of the diametric interference in meshing (patent RU 2194880, IPC F04C 2/16, F04C 5/00, 20 Dec. 2002].
A drawback of said design consists in that it does not take into account the conditions of sliding of the rotor's helical teeth on those of the stator, i.e. in the zone farthermost from the immediate centre of rotation (from the pitch point), where the sliding speeds are the greatest; and due to the evenly distributed interference there takes place a more severe wear of the resilient-elastic teeth of the stator and that of the rotor teeth's wear-resistance cladding. Another drawback consists in that the operation conditions of the gerotor mechanism are not taken into account (temperature, nature of the loads occurring in drilling of rocks of various hardness and composition); for example for the hot wells having a work temperature over 100° C., use of the gerotor mechanisms having a clearance in the rotor-stator meshing is required. The use, in such wells, of gerotor mechanisms having the in-meshing interference may result in a more severe wear, a sharp fall of efficiency and seizure of a mechanism. Another drawback of the known device is lack of possibility of varying the interference and of correlation adjustment of shapes of the rotor and stator teeth without changing the rotor and/or stator outer diameters, which does not allow to provide a reliable tightness along the contact lines in the gerotor mechanism, with zero interference in meshing.